Denver International Airport and the Regional Transportation District are in a legal tussle over which public entity is responsible for about $53 million worth of construction work at the new FasTracks train station set to open at the airport in January 2016.

DIA contends RTD should pay for the work because it was done to support construction of the train station. The station is part of RTD’s $1.17 billion East commuter rail line that will run between downtown’s Denver Union Station and DIA.

But it's also one element of DIA’s $544 million South Terminal project, which includes a 519-room hotel and conference space plus an open-air plaza between the hotel and the airport terminal.

The suit revolves around which side is responsible for the cost of doubling the length of the station platform from 400 feet to 800 feet, as well as some costs for excavation, retaining walls and bridge work at the site.

The site has been bustling with workers for months, and the Denver Business Journal took a tour of the site in October.

An independent arbitrator hired to help setting the suit, former Colorado Court of Appeals Judge Steve Briggs, ruled in June 2013 that RTD should pay about $7.8 million as its share of the disputed work.

Denver court filings say that decision unfairly “shifted tens of millions of dollars” worth of costs onto DIA’s side of the books.

An airport official said the total amount in dispute is about $53 million.

The city of Denver filed its lawsuit a month after Briggs’ decision, in July 2013, asking a district court judge to throw out the arbitrator’s ruling.

The city’s suit says the ruling:

• Isn’t supported by evidence presented during hearings.

• Was reached when the former judge misapplied Colorado contract law.

• And that the former judge held DIA to the incorrect burden of proof standard.

As for the work that needed to be done in order for the FasTracks construction project to stay on schedule — it was finished as of Dec. 31, the contractual date of the Intergovernmental Agreement between the two entities, according to DIA and RTD officials.

DIA’s and RTD’s differing interpretations of the agreement — specifically, parts that divvy up costs of the station — are at the core of the dispute.

Denver’s suit says the agreement called for DIA to pay only for the design and construction of the rail station, the terminal-to-station interface, excavation work, relocating utilities and building a road and drainage pond on the site.

And the suit says Denver disagrees with RTD’s interpretation that if the contract didn’t spell out specifics on which entity pays for what, then DIA should pay for the work.

RTD, in its court filing responding to DIA’s suit, says Denver is wrong to seek to dismiss Briggs’ decision.

RTD spokesman Scott Reed, in a statement sent via email, said DIA and RTD cooperated on the design and construction of the project — including the decision to keep construction work moving forward rather than fighting over the costs.

“Both DIA and RTD worked in a genuinely cooperative manner on design and construction of the commuter rail system to the airport and DIA’s hotel and transit center project, and the interface between the two projects,” Reed said.

“That cooperation is clearly displayed by the on-going progress of work at the site itself, where delay would only have added to costs for both parties and delayed use of the facilities by the public,” he said.